Various opinions have been entertained in regard to the formation, or growth, of bone. Some anatomists have supposed that all bone is formed in cartilage. But this is not true, for there is an _intra-membranous_, as well as an _intra-cartilaginous_, formation of bone, as may be seen in the development of the cranial bones, where the gradual calcification takes place upon the inner layers of the fibrous coverings.
Intra-cartilaginous deposit is found in the vicinity of the blood-vessels, within the cartilaginous canals; also, there are certain points first observed in the shafts of long bones, called _centers of ossification_. These points are no sooner formed than the cartilage corpuscles arrange themselves in concentric zones, and, lying in contact with one another, become very compact. As ossification proceeds, the cup-shaped cavities are converted into closed interstices of bone, with extremely thin lamellae, or layers. These, however, soon increase in density, and no blood-vessels can be observed within them.
[Illustration: Fig. 7.
Vertical section of cartilage near the surface of ossification. _1_. Ordinary appearance of the temporary cartilage. _1_'. Portion of the same more highly magnified. _2_. The cells beginning to form into concentric zones. _2_'. Portion more magnified.
_3_. The ossification is extending in the inter-cellular spaces, and the rows of cells are seen resting in the cavities so formed, the nuclei being more separated than above. _3_'. Portion of the same more highly magnified.]
[Illustration: Fig. 8.
Thigh-bone, sawn open lengthwise.]
[Illustration: Fig. 9.
Lower end of the thigh-bone sawn across, showing its central cavity.]
The bony plates form the boundaries of the _Haversian_, or nutritive canals of the bones. In the _second stage of ossification_, the cartilage corpuscles are converted into bone. Becoming flattened against the osseous lamellae already formed, they crowd upon one another so as to entirely obliterate the lines that distinguish them; and, simultaneously with these changes, a calcareous deposit takes place upon their interior. Bones grow by additions to their ends and surfaces. In the child, their extremities are separated from the body of the bone by layer of cartilage, and the cancellated, or cellular structure, which remains for a time in the interior, represents the early condition of the ossifying substances.
The bones contain more earthy matter in their composition than any other part of the human body, being firm, hard, and of a lime color. They compose the skeleton or frame work, and, when united by natural ligaments, form what is known as the _natural_ skeleton; when they are wired together, they are called an _artificial_ skeleton. The number of bones in the human body is variously estimated; for those regarded as single by some anatomists are considered by others to consist of several distinct pieces. There are two hundred distinct bones in the human skeleton besides the teeth. These may be divided into those of the Head, Trunk, Upper Extremities, and Lower Extremities.
[Illustration: Fig. 10.
The bones of the skull separated. _1_. Frontal, only half is seen. _2_. Parietal. _3_. Occipital, only half is seen. _4_. Temporal. _5_. Nasal. _6_. Malar.
_7_. Superior maxillary (upper jaw). _8_. Lachrymal.
_9_. Inferior maxillary (lower jaw). Between _4_ and _6_ a part of the sphenoid or wedge-shaped bone, is seen. Another bone assisting to form the skull, but not here seen, is called the _ethmoid_ (sieve-like, from being full of holes), and is situated between the sockets of the eyes, forming the roof of the nose.]
THE BONES OF THE HEAD are classed as follows: eight belonging to the Cranium, and fourteen to the Face. The bones of the Cranium are the _occipital_, two _parietal_, two _temporal, frontal, sphenoid_, and _ethmoid_. Those composing the face are, the two _nasal_, two _superior maxillary,_ two _lachrymal_, two _malar_ two _palate_, two _inferior turbinated, vomer_, and _inferior maxillary_. The cranial bones are composed of two dense plates, between which there is, in most places a cancellated or cellular tissue. The external plate is fibrous, the internal, compact and vitreous. The skull is nearly oval in form, convex externally, the bone being much thicker at the base than elsewhere, and it is, in every respect admirably adapted to resist any injury to which it may be exposed, thus affording ample protection to the brain substance which it envelops. The internal surface of the cranium presents eminences and depressions for lodging the convolutions of the brain, and numerous furrows for the ramifications of the blood-vessels.
The bones of the cranium are united to one another by ragged edges called _sutures_, which are quite distinct in the child but which in old age are nearly effaced. Some authorities suppose that by this arrangement the cranium is less liable to be fractured by blows; others think that the sutures allow the growth of these bones, which takes place by a gradual osseous enlargement at the margins. The bones of the _Face_ are joined at the lower part and in front of the cranium, and serve for the attachment of powerful muscles which assist in the process of mastication. Although the soft parts of the face cover the bony structure, yet they do not conceal its principal features, or materially change its proportions. The form of the head and face presents some remarkable dissimilarities in different races.
[Illustration: Fig. 11.
_1_. The first bone of the sternum (breast-bone).
_2_. The second bone of the sternum.
_3_. The cartilage of the sternum. _4_. The first dorsal vertebra (a bone of the spinal column). _5_. The last dorsal vertebra. _6_.
The first rib. _7_. Its head. _8_. Its neck. _9_.
Its tubercle. _10_. The seventh or last true rib. _11_. The cartilage of the third rib. _12._ The floating ribs.]
[Illustration: Fig. 12.
A vertebra of the neck. _1_. The body of the vertebra. _2_. The spinal canal. _4_. The spinous process cleft at its extremity. _5_. The transverse process. _7_. The interior articular process. _8_. The superior articular process.]
THE TRUNK has fifty-four bones, which are as follows: The _Os Hyoides_, the _Sternum_, twenty-four Ribs, twenty-four _vertebrae_ or bones of the Spinal Column, the _Sacrum_, the _Coccyx_, and two _Ossa Innominata_.
The _Os Hyoides_, situated at the base of the tongue, is the most isolated bone of the skeleton, and serves for the attachment of muscles.
The _Sternum_, or breast-bone, in a child is composed of six pieces, in the adult of three, which in old age are consolidated into one bone. The _Ribs_ are thin, curved bones, being convex externally. There are twelve on each side, and all are attached to the spinal column. The seven upper ribs, which are united in front of the sternum, are termed _true_ ribs; the next three, which are not attached to the sternum, but to one another are called _false_ ribs; and the last two, which are joined only to the vertebrae, are designated as _floating_ ribs. The first rib is the shortest, and they increase in length as far as the eighth, after which this order is reversed.
[Illustration: Fig. 13.
_1_. The cartilaginous substance which connects the bodies of the vertebrae. _2_. The body of the vertebra. _3_. The spinous process.
_4,4_. The transverse processes.
_5,5_. The articular processes.
_6,6_. A portion of the bony bridge which assists in forming the spinal canal (7).]
[Illustration: Fig. 14.
Backbone, spinal column, or vertebral column. All animals possessing such a row of bones are called _vertebrates_.
Above _b_ are the cervical (neck) vertebrae; _b_ to _c_, dorsal (back) or chest vertebrae; _c_ to _d_, lumbar (loins) vertebrae; _d_ to _e_, sacrum; _e_ to _f_, coccyx.]
The _Spinal Column_ or backbone, when viewed from the front presents a perpendicular appearance, but a side view shows four distinct curves.
The bones composing it are called _vertebrae_. The body part of a vertebra is light and spongy in texture, having seven projections called _processes_, four of which are the _articular_ processes, which furnish surfaces to join the different vertebrae of the spinal column. Two are called _transverse_, and the remaining one is termed the _spinous_. The transverse and spinous processes serve for the attachment of the muscles belonging to the back. All these processes are more compact than the body of the vertebra, and, when naturally connected, are so arranged as to form a tube which contains the _medulla spinalis_, or spinal cord.
Between the vertebrae is a highly-elastic, cartilaginous and cushion-like substance, which freely admits of motion, and allows the spine to bend as occasion requires. The natural curvatures of the spinal column diminish the shock produced by falling, running or leaping, which would otherwise be more directly transmitted to the brain. The ribs at the sides, the sternum in front, and the twelve dorsal bones of the spinal column behind, bound the thoracic cavity, which contains the lungs, heart, and large blood-vessels.
[Illustration: Fig. 15.
A representation of the pelvic bones. _e_. The lumbo-sacral joint. 2. The sacrum. _3_. Coccyx. _1,1_.
The innominata. _4,4_. Acetabula.]
The _Pelvis_ is an open bony structure, consisting of the Os Innominata, one on either side, and the Sacrum and Coccyx behind. The _Sacrum_, during childhood, consists of five bones, which in later years unite to form one bone. It is light and spongy in texture, and the upper surface articulates with the lowest vertebra, while it is united at its inferior margin to the coccyx. The _Coccyx_ is the terminal bone of the spinal column. In infancy it is cartilaginous and composed of several pieces, but in the adult these unite and form one bone. The _Innominata_, or nameless bones, during youth, consist of three separate pieces on each side; but as age advances they coalesce and form one bone. A deep socket, called the _acetabulum_, is found near their junction, which serves for the reception of the head of the thigh-bone.
[Illustration: Fig. 16.
1. Portions of the backbone. 2. Cranial bones. _4_. Breast-bone. _5_. Ribs. _7_. Collar-bone.
_8_. Arm-bone (humerus). _9_. Shoulder-joint.
_10, 11_. Bones of the fore-arm (ulna and radius). _12_. Elbow-joint. _13_. Wrist-joint. _14_.
Bones of the hand. _15, 16_. Pelvic bones. _17_.
Hip-joint. _18_. Femur. _19, 20_. Bones of the knee-joint. _21, 22_. Fibula and tibia. _23_. Ankle bone. _24_. Bones of the foot.]
THE BONES OF THE UPPER EXTREMITIES are sixty-four in number, and are classified as follows: The Scapula, Clavicle, Humerus, Ulna, Radius, Carpus, Metacarpus, and Phalanges. The _Scapula_, or shoulder-blade, is an irregular, thin, triangular bone, situated at the posterior part of the shoulder, and attached to the upper and back part of the chest. The _Clavicle_, or collar-bone, is located at the upper part of the chest, between the sternum and scapula, and connects with both. Its form resembles that of the italic letter _f_, and it prevents the arms from sliding forward. The _Humerus_, the first bone of the arm, is long, cylindrical, and situated between the scapula and fore-arm. The _Ulna_ is nearly parallel with the radius, and situated on the inner side of the fore-arm. It is the longer and larger of the two bones, and in its articulation with the humerus, forms a perfect hinge-joint. The _Radius_, so called from its resemblance to a spoke, is on the outer side of the fore-arm, and articulates with the bones of the wrist, forming a joint. The ulna and radius also articulate with each other at their extremities. The _Carpus_, or wrist, consists of eight bones, arranged in two rows. The _Metacarpus_, or palm of the hand, is composed of five bones situated between the carpus and fingers. The _Phalanges_, fourteen in number, are the bones of the fingers and thumb, the fingers each having three and the thumb two.
THE BONES OF THE LOWER EXTREMITIES, sixty in number, are classed as follows: The Femur, Patella, Tibia, Fibula, Tarsus, Metatarsus, and Phalanges. The _Femur_, or thigh-bone, is the longest bone in the body.
It has a large round head, which is received into the acetabulum, thus affording a good illustration of a ball and socket joint. The _Patella,_ or knee-pan, is the most complicated articulation of the body. It is of a round form, connects with the tibia by means of a strong ligament, and serves to protect the front of the joint, and to increase the leverage of the muscles attached to it, by causing them to act at a greater angle. The _Tibia_, or shin bone, is enlarged at each extremity and articulates with the femur above and the astragalus, the upper bone of the tarsus, below. The _Fibula_, the small bone of the leg, is situated on the outer side of the tibia, and is firmly bound to it at each extremity. The _Tarsus_, or instep, is composed of seven bones, and corresponds to the carpus of the upper extremities. The _Metatarsus_, the middle of the foot, bears a dose resemblance to the metacarpus, and consists of five bones situated between the tarsus and the phalanges.
The tarsal and the metatarsal bones are so united as to give an arched appearance to the foot, thus imparting elasticity. The _Phalanges_, the toes, consist of fourteen bones, arranged in a manner similar to that of the fingers.
We are not less interested in tracing the formation of bone through its several stages, than in considering other parts of the human system. The formation of the Haversian canals for the passage of blood-vessels to nourish the bones, the earlier construction of bony tissue by a metamorphosis of cartilaginous substance, and also the commencement of ossification at distinct points, called _centers of ossification_, are all important subjects, requiring the student's careful attention. The bones are protected by an external membranous envelope, which, from its situation is called the _periosteum_. The bones are divided into four classes, _long, short, flat_ and _irregular_, being thus adapted to subserve a variety of purposes.
The Long Bones are found in the limbs, where they act as levers to sustain the body and aid in locomotion. Each_long_ bone is composed of a cylinder, known as the _shaft_, and two _extremities_. The shaft is hollow, its wails being _thickest_ in THE middle and growing thinner toward the extremities. The _extremities_ are usually considerably enlarged, for convenience of connection with other bones, and to afford a broad surface for the attachment of muscles. The clavical, humerus, radius, ulna, femur, tibia, fibula, the bones of the metacarpus, metatarsus and the phalanges, are classed as long bones.
Where the principal object to be attained is strength, and the motion of the skeleton is limited, the individual bones are short and compressed, as the bones of the carpus and tarsus. The structure of these bones is spongy, except at the surface, where there is a thin crust of compact matter.
[Illustration: Fig. 17.
Anatomy of a joint, _1, 1_.
Bones of a joint. _2, 2_. Cartilage.
_3, 3, 3, 3_. Synovial membrane.]
[Illustration: Fig. 18.
Anatomy of knee joint.
_1._ Lower end of thigh-bone.
_3._ Knee-pan. _2, 4_ Ligaments of the knee-pan. _5_. Upper end of the tibia, or shin-bone.
_6, 12_. Cartilages.]
When protection is required for the organs of the body, or a broad flat surface for the attachment of the muscles, the bones are expanded into plates, as in the cranium and shoulder-blades.
The _irregular_ or _mixed_ bones are those which, from their peculiar shape, cannot be classed among any of the foregoing divisions. Their structure is similar to the others, consisting of cancellar tissue, surrounded by a crust of compact matter.
The vertebrae, sacrum, coccyx, temporal, sphenoid, ethmoid, malar, two maxillary, palate, inferior turbinated, and hyoid are known as irregular bones.
The formation of the joints requires not only bones, but also cartilages, ligaments, and the synovial membrane, to complete the articulation. _Cartilage_ is a smooth, elastic substance, softer than bone, and invested with a thin membrane, called _perichondrium_. When cartilage is placed upon convex surfaces, the reverse is true. The _Ligaments_ are white, inelastic, tendinous substances, softer than cartilage, but harder than membrane. Their function is to bind together the bones. The _Synovial Membrane_ covers the cartilages, and is then reflected upon the ligaments, thus forming a thin, closed sac, called the _synovial capsule._
All the synovial membranes secrete a lubricating fluid, termed _synovia_, which enables the surfaces of the bones and ligaments to move freely upon one another. When this fluid is secreted in excessive quantities, it produces a disease known as "dropsy of the joints." There are numerous smaller sacs besides the synovial, called _bursae mucosae_, which in structure are analogous to them, and secrete a similar fluid.
Some joints permit motion in every direction, as the shoulders, some in two directions only, as the elbows, while others do not admit of any movement. The bones, ligaments, cartilages, and synovial membrane, are supplied with nerves, arteries, and veins.